In a ruling released on Monday for Heien v. North Carolina, the Supreme Court decided with an 8-1 vote that police are justified in stopping citizens for actions they "reasonably" believe are illegal, even if the officers' understanding of the law is significantly or entirely mistaken.
The specific case under consideration concerned a traffic stop over a broken tail light, which led to discovery of drugs in the car. But since North Carolina only requires drivers to have a single "stop lamp," the driver arguably should never have been stopped at all.
In the lone dissenting opinion, Justice Sonia Sotomayor wrote, "One is left to wonder why an innocent citizen should be made to shoulder the burden of being seized whenever the law may be susceptible to an interpretative question." The full majority and dissenting opinions are available here.